1. Field of the Invention
The present invention relates to an inkjet recording method and an inkjet recording apparatus, and more particularly to an inkjet recording method and an inkjet recording apparatus based on a direct printing method which forms an image by directly depositing aqueous ink onto a recording medium.
2. Description of the Related Art
An inkjet recording apparatus is able to record images of good quality by means of a simple composition, and therefore such apparatuses are widely used as domestic printers for individual use and office printers for commercial use. In the case of office printers for commercial use, in particular, there are increasing demands for higher processing speed and higher image quality.
In improving the image quality achieved by an inkjet recording apparatus, generally, it is necessary that there should be little interference between ink droplets ejected from the nozzles of the ink head (hereinafter referred to as “landing interference”), little contraction of the image (hereinafter referred to as “image contraction”) and good reproducibility of text characters (hereinafter referred to as “text reproducibility”), and so on.
Moreover, in the inkjet recording apparatus, it is also necessary to suppress curl, and the like, and to improve the image strength. In other words, if water is used as a solvent in the ink, then the water permeates into the recording medium, deformation of the recording medium, such as curling or cockling, is liable to occur, and therefore suppressing of curl, and the like, is required. Furthermore, if general paper such as offset printing paper is used as the recording medium, then the image becomes more liable to disturbance when the paper is rubbed and therefore “image strength” is required.
Various methods have been proposed in response to requirements of these kinds. For example, Japanese Patent Application Publication No. 2004-010633 discloses an image forming method in which a powder layer is deposited on an intermediate transfer body, this powder layer causes the ink to swell, rise in viscosity and separate by reaction with the ink, and is then transferred to the recording medium. According to this method, it is possible to form an image that produces little bleeding on the recording medium.
However, Japanese Patent Application Publication No. 2004-010633 uses an indirect printing method, which first forms an image (ink aggregate body) on the intermediate transfer body and then transfers this image to a recording medium, and therefore has a problem in that a greater number of steps are involved compared to a direct printing method, which forms an image directly onto a recording medium, and a problem in that the apparatus becomes correspondingly more complicated. Consequently, a method which is compatible with a direct printing method is required, and a method using special inks has been proposed as one example of such a method. For example, Japanese Patent Application Publication No. 11-188858 discloses an image forming method using an ink set including an aqueous ink containing pigment, water-soluble solvent and water, and a liquid composition that causes the aqueous ink to aggregate, wherein by making one of the aqueous ink and the liquid composition alkaline and the other acidic, it is possible to achieve excellent recording in terms of optical density, bleeding and color bleeding.
Japanese Patent Application Publication No. 2000-037942 discloses technology for improving optical density, bleeding, color mixing and drying duration, by controlling the aggregating properties of pigment on a recording medium through making one of a liquid composition (treatment liquid) and ink acidic and making the other alkaline.
Japanese Patent Application Publication No. 2007-161753 discloses an image forming method using an ink set including a colorless ink and colored ink, in which the total weight of water-soluble organic solvent contained in the ink is not smaller than 50 wt % and not larger than 90 wt % of the total weight of the ink, and furthermore 30 wt % of the high-boiling-point solvent has an SP value not less than 16.5 and not more than 24.6. According to this method, it is possible to reduce print-through and curl, as well as improving wear ability and ejection stability.
Japanese Patent Application Publication No. 2007-175922 discloses an image forming method using an ink set which includes a pigment, a water-soluble organic solvent and water, the water-soluble organic solvent having an SP value of not lower than 16.5 and lower than 24.6 accounting for 30 wt % or more of the total weight of ink. According to this method, it is possible to prevent curling and cockling when printing in a single pass.
However, even if the inks in the related art are used, it is not possible to sufficiently satisfy all of the conditions of improving image quality, suppressing curl and improving image strength. For example, if the inks in the related art are deposited on a recording medium in an inkjet recording apparatus using a direct printing method, then there are problems in that curl is liable to occur if the deposition volume per unit time is raised, and image strength declines if a normal paper is used as the recording medium.
In view of these circumstances, there is demand for an image forming method using a direct printing system of forming images by applying a aqueous ink directly onto a recording medium by means of an inkjet recording apparatus, which satisfies the conditions of producing little landing interference or image contraction, having good text reproducibility and making curl not liable to occur. In particular, in an office printer, general papers such as art paper or copy paper, or the like are commonly used as recording media, in addition to special papers such as coated papers, and therefore it is necessary to satisfy all of the conditions described above, regardless of the type of recording medium.